A membrane separation activated sludge process, which is a type of wastewater treatment method, consists of submerging a membrane cartridge in an activated sludge tank and carrying out solid-liquid separation to separate the activated sludge and treated liquid by filtration. Since this method allows solid-liquid separation to be carried out at an extremely high activated sludge (mixed liquor suspended solid: MLSS) concentration of 5000 to 20000 mg/l, it offers the advantages of being able to reduce the volume of the activated sludge tank or shorten the reaction time in the activated sludge tank. In addition, since suspended solids (SS) do not contaminate the treated water due to the use of membrane filtration, a final sedimentation tank is not required, the area of the treatment facility site can be reduced and solid-liquid separation can be carried out regardless of the ease of settling of the activated sludge, thereby resulting in rapid proliferation of this process in recent years since it offers numerous advantages such as being able reduce the burden of activated sludge management.
Flat sheet membranes or hollow fiber membranes are used for the membrane cartridge. In a membrane separation activated sludge process, since the effective membrane surface area is reduced resulting in a decrease in filtration efficiency due to the adherence to the membrane surface of biopolymers metabolized by microorganisms in the activated sludge, the activated sludge itself or contaminants contained in the wastewater, there are cases in which it is difficult to maintain stable filtration for a long period of time. At this time, backwashing may be carried out in which a medium such as filtered water is effused in the opposite direction from the direction of filtration to remove adhered substances on the surface of the membrane.
In the past, in order to avoid this accumulation of activated sludge aggregates, contaminants and the like on the membrane surface and between the membranes, the membranes were aerated with air and the like from the bottom of the membrane cartridge, thereby causing activated sludge aggregates, contaminants and the like to be separated from the membrane surface and between the membranes due to vibration effects of the membranes and agitation effects produced by the upward movement of air bubbles. For example, Japanese Patent Application Laid-open No. 2000-157846 (Patent document 1) discloses a hollow fiber membrane cartridge in which a cartridge head is fastened to be liquid-tight to the outer periphery of one end of a hollow fiber bundle and a skirt is fastened to be liquid-tight to the outer periphery of the other end to allow the hollow fiber membrane to vibrate at the maximum amplitude within an allowed range during aeration, a hollow portion of the end of the hollow fiber membrane on the side of the cartridge head is open, a hollow portion of the end of the hollow fiber membrane on the side of the skirt is sealed, and a plurality of through holes are provided in an adhesive fixing layer on the side of the skirt.
However, stable solid-liquid separation may not be able to be carried out depending on the composition of the organic wastewater flowing into the activated sludge tank even if aeration and backwashing are carried out unless activated sludge treatment conditions are set properly. This is thought to be because numerous components are excreted that cause microorganisms to clog the membrane.
On the other hand, susceptibility to clogging can be reduced by increasing the concentration of the activated sludge, reducing the amount of organic matter that flows into the activated sludge, or setting a lower membrane filtration permeation flux. However, excessive use of such methods has the problem of decreasing the efficiency of wastewater treatment.    Patent document 1 Japanese Patent Application Laid-open No. 2000-157846